The invention relates to a multilayer wiring substrate.
An example of a conventional multilayer wiring substrate is described in U.S. Pat. No. 4,245,273. In the substrate proposed in the U.S. Patent, a set of signal planes 58, 60 and 64 with ceramic insulating layers inserted therebetween are sandwiched between ground planes 54 and 56. The ground planes 54 and 56 are provided so as to eliminate the cross-talk between signal lines in the signal planes 58, 60 and 64, and to make uniform the characteristic impedances of the lines. The characteristic impedance Z.sub.0 of a signal line is given by Z.sub.0 =(L.sub.0 /C.sub.0).sup.1/2, where L.sub.0 and C.sub.0 denote the inductance and capacitance of the signal line, respectively. The signal-propagation delay time T.sub.0 of the line is given by T.sub.0 =(L.sub.0 /C.sub.0).sup.1/2. The capacitance of the signal line C.sub.0 is proportional to the dielectric constant .epsilon. of insulating layers in contact with the line, while the signal-propagation delay time T.sub.0 is proportional to (.epsilon.).sup.1/2. Since the delay time T.sub.0 is the time required for an electric pulse to transmit along a signal path, it is preferable for it to be as short as possible. Thus, the magnitude of the dielectric constant .epsilon. is favored to be small. On the other hand, a desirable value of the characteristic impedance Z.sub.0 is in the range from 40 to 80 ohms. It depends strongly upon the signal line width, and upon the dielectric constant of the insulating layers. Since the line width is limited by a required direct-current resistance of the signal line and by the density of signal lines per unit area, it is difficult to choose the line width freely. Moreover, since the dielectric constant is determined by the insulating material used, the value of the characteristic impedance Z.sub.0 is difficult to choose freely.
An object of the invention is, therefore, to provide a multilayer wiring substrate free from the above-mentioned disadvantages.